The present invention relates to a system of self-diagnosing exhaust gas control components/systems for an automobile engine. More specifically, the invention relates to a control procedure (logic) of a self-diagnosing function portion which has a function of checking a failure/malfunction of those components/systems in an intensive manner.
As the issue of earth environments becomes more significant, stricter regulations are now being enforced on exhaust gas produced from automobiles. With this trend, it is now necessary to provide a function of monitoring and checking whether components and systems relating to the exhaust gas control are working normally. Among those components and systems are a purge air control system for controlling transpired gas (hereinafter referred to as "purge air") from a fuel tank, an ignition failure detection system for monitoring whether the combustion of an engine is normal, and an 02 feedback control system for properly effecting the cleaning by a catalyzer.
For example, Japanese Patent Application Unexamined Publication No. Hei. 2-130255 discloses a conventional diagnosing device that is provided for an automobile engine to check its purge air control system. This diagnosing device checks malfunction of the purge air control system independently of failure/malfunction checks on other exhaust gas control components/systems, such as an ignition system (ignition failure), an O.sub.2 sensor and a fuel supply system.
FIG. 1 shows the entire system relating to the conventional purge air control. Reference numeral 1 denotes a fuel tank; 2, a pressure sensor for detecting pressure in the fuel tank 1; and 3, a canister having activated carbon for absorbing purge air coming from the fuel tank 1. A solenoid A serves to open/close an air passageway 4 for passing purge air to the atmosphere. A solenoid B serves to open/close an engine-side passageway 5, which is connected to an engine intake pipe 20. Reference numeral 6 denotes an automobile engine; 7, an engine control computer unit (ECU) for controlling the engine 6; 8, injectors for supplying fuel to the engine 6; 9, an O.sub.2 sensor attached to an engine exhaust pipe 21; 10, a crank shaft sensor for measuring a crank shaft angle; 11, a water temperature sensor; 12, a fuel pressure regulator; and 13, a fuel pump.
With reference to FIG. 6, a description will be made of a conventional operation of checking for malfunctions of the purge air control system. Purge air accumulated in the space of the fuel tank 1 is absorbed by activated carbon provided in the canister 3. The air passageway 4 of the canister 3 is usually opened to the atmosphere, and serves as an emergency passageway to discharge purge air from the canister 3 only when an abnormally large amount of purge air is absorbed in the canister 3.
The ECU 7 monitors information sent from sensors attached to respective parts of the engine 6. If the ECU 7 judges that the engine operation state is such that the canister 3 is capable of absorbing purge air, it establishes a purge check mode (at time T.sub.0) and turns off the solenoids A and B to close the air passageway 4 of the canister 3 and the engine-side passageway 5, to thereby seal the purge air passageway. With no places for escape, the space of the fuel tank 1 is filled with purge air to increase the pressure in the fuel tank 1. When this state has continued for a predetermined period (until time T.sub.1 ; the tank internal pressure increases up to P.sub.0), the solenoid B is turned on to discharge the purge air filling the canister 3 to the engine intake pipe 20 for a predetermined period (until time T.sub.2). As a result, the fuel tank internal pressure decreases to P.sub.1.
Then, the solenoid B is turned off to again close the purge air passageway, and a period (t.sub.m) necessary for the tank internal pressure to make a predetermined increase (.DELTA.P.sub.2) is measured.
If the purge air control system is operating normally, the tank internal pressure changes in a manner as indicated by a solid line in FIG. 6, in which case the period t.sub.m is measured as t.sub.2. On the other hand, if there exists a leak of purge air due to, for instance, damage of the solenoid A or B or the purge air passageway anywhere between the fuel tank 1 and the engine intake pipe 20, the tank internal pressure changes in a manner as indicated by a broken line in FIG. 6, in which case the period t.sub.m is measured as t.sub.1. That is, the pressure increases more slowly.
In the above manner, malfunction of the purge air control system can be checked based on the length of the pressure increase period t.sub.m.
Next, a conventional operation of checking an ignition failure will be described with reference to FIG. 7
The ECU 7 detects the rotation speed of the engine 6 by measuring the period of a signal sent from the crank shaft sensor 10. When an ignition failure occurs in the engine 6 at time T.sub.100 in FIG. 7, no torque is generated by a cylinder of the ignition failure. Therefore, the rotation speed of the crank shaft decreases to elongate the period of the signal sent from the crank shaft sensor 10. That is, the period increases to T.sub.B1 at time T.sub.200. It is judged that an ignition failure has occurred when T.sub.B1 exceeds an ignition failure judgment level T.sub.B2. Thus, a failure in ignition components can be detected.
Next, with reference to FIG. 8, a description will be made of a conventional operation of checking a failure of the O.sub.2 sensor 9.
The top part of FIG. 8 shows the onset of an O.sub.2 sensor failure check mode, the middle part shows a waveform of an output signal of the O.sub.2 sensor 9, and the bottom part shows a control signal sent from the ECU 7 to the injector 8 and indicating the amount of fuel to be supplied to the engine 6.
In FIG. 8, the usual O.sub.2 feedback control is performed until time T.sub.20. That is, if the output signal of the O.sub.2 sensor indicates a rich state (the air-fuel ratio A/F is smaller than 14.7), the amount of fuel supplied to the engine 6 is decreased. Conversely, if the output signal of the O.sub.2 sensor indicates a lean state (A/F is larger than 14.7), the amount of fuel is increased. Thus, the fuel supply amount is adjusted to reverse the output signal of the O.sub.2 sensor 9.
When the ECU 7 judges (at time T.sub.20 ) that the engine 6 is in such a state that the O.sub.2 sensor failure check mode should be established, the ECU 7 produces an instruction to keep the fuel supply amount at a predetermined lower level (F.sub.1) for a predetermined period (time T.sub.20 to T.sub.21), and an instruction to thereafter keep it at a predetermined higher level (F.sub.2) for another predetermined period (time T.sub.21 to T.sub.22).
If the O.sub.2 sensor 9 is normal, its output signal is decreased to a level V.sub.L1 at time T.sub.21 (the end of the lean period), and thereafter increased in response to the increase of the fuel supply amount to reach a predetermined judgment level V.sub.th after a lapse of a period t.sub.h1. On the other hand, if the O.sub.2 sensor 9 is deteriorated, common results are such that the response output voltage has a lower value or a delay. Therefore, when a deteriorated O.sub.2 sensor 9 is mounted on the engine 6, there may occur such a case that the sensor output voltage decreases only to a level V.sub.L2 at time T.sub.21 or it takes a longer period t.sub.h2 to reach the judgment level V.sub.th. The deterioration of the O.sub.2 sensor 9 can be detected based on those values.
Next, with reference to FIG. 9, a description will be made of a conventional operation of checking malfunction of the fuel supply system.
In a system in which the O.sub.2 feedback control is performed, the output voltage of the O.sub.2 sensor is larger than 0.5 V when the air-fuel ratio (A/F) is smaller than 14.7 (rich state), and is smaller than 0.5 V when A/F is larger than 14.7 (lean state). In general, to maintain the air-fuel ratio of 14.7, which is most appropriate in terms of the exhaust gas performance, the amount of fuel supplied to the engine 6 is controlled so as to reverse the output voltage of the O.sub.2 sensor, as was described above in connection with FIG. 8.
In the system under consideration, an O.sub.2 feedback correction coefficient is used so as to realize an integration type correction in which the fuel supply amount is changed gradually with respect to time elements as shown in FIG. 9. If respective components of the fuel supply system are normal (until time T.sub.40 in FIG. 9), usually the O.sub.2 feedback correction coefficient varies in the vicinity of 1.0. However, if there exists in the fuel supply system a deteriorated component (e.g., an injector 8) whose characteristics are different than the normal one, the above-described O.sub.2 feedback control is effected to correct the fuel supply amount so that the air-fuel ratio is kept at 14.7, to thereby compensate for differences (due to the deterioration) of the characteristics. As a result, the O.sub.2 feedback correction coefficient shifts as shown in FIG. 9 (after time T.sub.41). Thus, a judgment can be made of the deterioration degree of the components relating to the fuel supply system based on whether the shift exceeds a predetermined range (-I.sub.th to +I.sub.th in FIG. 9).
As was described above in connection with FIG. 6, in the conventional operation of checking malfunction of the purge air control system, the purge air passageway is sealed from time T.sub.0 to T.sub.1 to force purge air of the fuel tank 1 to be accumulated in the canister 3, and then the accumulated purge air is suddenly supplied to the intake pipe 20 of the engine 6.
In many cases, the forcibly accumulated purge air has a small air-fuel ratio (rich). If such purge air is suddenly introduced into the engine 6, the engine combustion will temporarily be rendered in an overrich state. Depending on the operation state of the engine 6, this may cause an ignition failure. If the ignition failure checking operation of FIG. 7 is effected in this state, the ignition failure due to the overrich combustion will be detected, which results in a misjudgment that there exists a failure in ignition components.
Similarly, a problem will occur if the operation of checking an O.sub.2 sensor failure (see FIG. 8) is effected while the forcibly accumulated purge air is introduced into the engine 6 (time T.sub.1 to T.sub.2 in FIG. 6) in checking malfunction of the purge air control system. In this case, the actual air-fuel mixture will not become lean though the amount of fuel supplied to the engine 6 is reduced from time T.sub.20 to T.sub.21 (see FIG. 8) to make the air-fuel mixture lean. Since the O.sub.2 sensor 9 does not produces a lean output signal, there will occur a misjudgment that the O.sub.2 sensor 9 has failed.
A problem also occurs if the operation of checking malfunction of the fuel supply system (see FIG. 9) is effected while the forcibly accumulated purge air is introduced into the engine 6 (time T.sub.1 to T.sub.2 in FIG. 6) in checking malfunction of the purge air control system. In this case, the air-fuel mixture temporarily becomes overrich even though the components of the fuel supply system are normal and the correction coefficient of the O.sub.2 feedback control is close to 1.0. To compensate for this, the correction coefficient is shifted to the lean side as shown in FIG. 9. If the correction coefficient goes beyond the worst level (-I.sub.th), there will occur a misjudgment that the fuel supply system is malfunctioning.
In summary, the above-described misjudgments of the ignition failure, O.sub.2 sensor failure and malfunction of the fuel supply system are caused by the air-fuel mixture made temporarily overrich due to the sudden introduction of the purge air filling the canister 3 into the engine during the operation of checking malfunction of the purge air control system.